There is much interest in renewable power generation, particularly fuel cells. A fuel cell is an energy conversion device that invariably comprises two electrodes, i.e., a cathode and an anode, upon which electrochemical reactions occur to enable the transformation of stored chemical energy into electrical energy. Fuel (e.g., hydrogen, methanol) is oxidized at the anode to release electrons that are then routed through an external circuit, while protons are transported through a proton exchange membrane to the cathode. The circuit is completed inside the fuel cell when the electrons are received back again from the external circuit at the cathode, where they combine with protons and oxygen atoms in a reduction reaction to produce water. The output of the fuel cell then is electrical energy and heat, produced by the production of water from protons and oxygen.
Typically, catalysts are incorporated into the anode and cathode electrodes to facilitate fuel oxidation and oxygen reduction. Current preferred catalysts for fuel cells are particulate noble metals; however, these metals are expensive, inherently inefficient, non-renewable and not easily characterized. For these reasons, substitution of noble metals with homogeneous redox catalysts is a desirable goal; but, low current densities (which result in inadequate power and/or volume) have made this approach uneconomical with previously disclosed systems.
Use of enzymatic catalysts permit the incorporation of the redox catalyst into fluidic microdomains and thereby makes higher current densities possible due to: 1.) locally high concentrations of catalyst (since the catalyst is not confined to one monolayer density); 2.) high electron diffusion coefficients; and, 3.) opportunities for convective transport. Additionally a redox catalyst that is enzymatic can easily use bio-available energy sources such as glucose.
The redox enzyme in an enzymatic biofuel cell participates in an electron transfer chain at the anode by oxidizing the fuel. However, redox enzymes are incapable of direct contact with the electrode since their redox centers are insulated from the conductive support by their protein matrices (Katz et al., “Biochemical fuel cells”, In Handbook of Fuel Cells—Fundamentals, Technology and Applications, 1, Ch. 21 (2003)). In order to bring these enzymes into contact with the electrode and to improve the electron transfer rate, an initially oxidized electron transport redox mediator is used to reoxidize the enzyme. The electrons are then transferred to the anode and the mediator is once again oxidized. A similar process occurs at the cathode.
Numerous examples of enzymatic biofuel cells are described in the literature; however, existing work in this area either does not utilize the redox catalyst within the pores of the electrode or does not allow for unconstrained mobility of the electrochemical reagents within the pores of the electrode. For example, U.S. Pat. No. 4,224,125 teaches the use of oxidoreductase enzymes and electron transport mediators (ETMs) immobilized in the neighborhood of a current collector. However, the ETM is in the form of a redox polymer, and the redox catalyst is immobilized. EP 0,177,743 B1 provides for an immobilized redox catalyst and an ETM coated onto a carrier that forms part of the electrode. But, the electrochemical reaction occurs within a fluid volume between a film coated over the electrode external surface and the macroscopic surface of the electrode and not throughout the electrode volume. U.S. Pat. No. 6,294,281 teaches the use of enzymes as fuel cell catalysts together in both the anode and the cathode of a biological fuel cell by immobilization of the enzymes within a silica gel and subsequent application to an electrode surface in combination with a redox hydrogel that functions as an ETM. Electrodes like those described in U.S. Pat. No. 4,224,125 and EP 0 177743 B1 find use in sensor applications, but immobilization of the catalyst and/or ETM and restricted access of the electrochemical reagents to the redox catalyst immobilized inside the electrode limit the current densities that can be obtained in fuel cell applications. Fuel cells like those described in U.S. Pat. No. 6,294,281 are designed for use in vivo at low power and also produce current densities that are limited by the rates of electron and electrochemical reagent transport.
The open literature describes enzymatic biofuel cells that suffer from similar deficiencies to those described above, where commercial practicability is limited due to low current densities, resulting from immobilization of the redox catalyst and/or ETM and restricted access of the electrochemical reagents to the redox catalyst immobilized inside the electrode. For example:                Habermuller et al. (Fresenius J Anal Chem, 2000, 366:560-568) describe various electrode architectures for use in amperometric biosensors. They discuss: 1.) problems with electron transfer between electrode and enzyme; and 2.) how immobilization, monolayer formation and ETM diffusion can all contribute to the total current obtained.        Barton et al. (J. Phys. Chem. B., 2001, 105(47):11917-11921 and J. Amer Chem Soc. 2001, 123:5802-5803) describe a laccase cathode consisting of laccase immobilized in a non-fluidic redox polymer with no domain structure. Evidence is presented for transport-limited currents.        Palmore et al. (J. Electroanalytical Chem., 1999, 464:110-117) describe a biofuel cell utilizing laccase as the cathode electrocatalyst with ABTS as ETM. The cathode is an entirely homogeneous solution with no enzyme or ETM confinement and no microdomain formation.        
Tsujimura et al. (Phys. Chem. Chem. Phys., 2001, 3:1331-1335) describe a biofuel cell using carbon felt electrodes with a laccase/ABTS homogeneous solution as the cathode electrolyte, and bacterial cells that metabolize H2 on the anode. They demonstrate fuel cell performance but no method for microdomain formation.
A. A. Karyakin et al. (Electrochemistry Communications, 2002, 4:417-420) describe a fuel cell anode wherein hydrogenase from T. roseopersicina is immobilizzed at the surface of “carbon filament material”, probably carbon paper or cloth. The electrode does not employ an ETM and does not describe domains. Because of the monolayer interaction at the electrode surface, the current is limited.                S. V. Morozov et al. (Bioelectrochemistry, 2002, 55:169-171) describe a similar system to that of Karyakin et al. (supra), except the enzyme is immobilized but not in fluidic media, and there are no domains. Electron transfer rates are slow because of a polymeric ETM.        Chen et al. (J. Am. Chem. Soc., 2001, 123:8630-8631) describe a biofuel cell based upon enzyme systems at both the cathode and anode. The enzyme is immobilized in a redox polymer. The electron transfer rates are again slow because of a polymeric ETM.        Katz et al. (J. Electroanalytical Chem., 1999, 479:64-68) describe a biofuel cell consisting of monolayered enzyme cathode and anode. There is no description of fluidic domains, and the current is limited by monolayer coverage.        Trudeau et al. (Analytical Chemistry, 1997, 69:882-886) describe covalent immobilization of laccase and ETM in a hydrogel. The laccase and ETM are bound and the domains are not defined. The immobilization limits the effective concentration of laccase and ETM and limits the rate of electron transfer.        Willner et al. (Bioelectrochemistry and Bioenergetics, 1998, 44:209-214) describe a biofuel cell with electrodes that utilize covalently linked ETMs and free enzyme in solution. Domains are not indicated and the ETM is immobilized covalently.        Katz et al. (New J of Chemistry, 1999, 5:481-487) describe a similar biofuel cell to that of Willner et al. (supra), except that the enzyme is covalently linked through a molecular wire onto the electrode.        Nakagawa et al. (Chemistry Letters 2003, 32:54-55) describe an enzyme electrode using bilirubin oxidase and [Fe(CN)6]3-/4- as enzyme and ETM. Both are trapped on a glassy carbon electrode. No domain structure and no porous electrode is indicated. Diffusion-limited currents are obtained.Each of the electrodes described above are useful in small fuel cell applications; however, all suffer from the deficiency of low current densities and are not commercially practicable.        
Thus, a need exists for the development of a redox catalyst- or enzymatic redox catalyst-based electrode capable of generating useful current densities. Applicants have solved the stated problem by the design of a fuel cell electrode that comprises the redox catalyst and substrate in fluid association with each other within a microdomain of the electrode.